THE SKELETON AND TEETH 109
palate bone floor the quadrilateral posterior nares or choanse.
The part of the basis cranii behind the palatal region .is formed by parts of the sphenoid, temporal and occipital bones. The parts of the sphenoid visible are the strong basi-sphenoid, which is united to the basi-occiput by a marked horizontal suture, the presphenoid and the four pterygoid plates. The pterygoid hamuli are slender and sharp, and the pterygoid fossae are wide. . No tympanic bullse are present, but the Eustachian processes are well marked. Styloid and vaginal processes are absent.
The- Cranial Cavity.—The cranial capacity varies from 76 to 90 c.c. ; and sexual differences are negligible. The anterior fossa is much reduced in size by the upward projection of the dome-like roofs of the orbits. So much are the orbital plates of the frontal bone bulged up that the cribriform plate of the e'thmoid bone is at the bottom of a deep sulcus. The clinoid processes may be fused, and the long axis of the sella turcica is horizontal. The petrous temporal has a cerebellar pit, and the grooves for the lateral sinuses, particularly the right one, are well marked.
As regards the foramina it is important to note that the foramen ovale pierces the alisphenoid; and the foramen magnum lies far back, being partly on the base and partly on the occipital plane of the skull.
Mandible.—The two halves are united in front at an acute angle. The posterior aspect of the symphysis menti has a fossa for the origin of the genial muscles. The symphysis is receding in the genus Hylobates, but is more vertical in Symplialangus. The ascending ramus